1. Field of the Invention
The present invention relates to a magnetostrictive torque detecting apparatus, for detecting torque applied to a shaft made of a magnetostrictive material.
2. Description of the Prior Art
The magnetostrictive torque detecting apparatus as disclosed in the above-mentioned U.S. Patent document and Japanese Published Unexamined (Kokai) Pat. Appli. No. 62-185136, comprises a shaft made of a magnetostrictive material, a pair of magnetizing coil and detecting coils loosely arranged around the shaft, and a yoke for housing the two coils so as to form a magnetic circuit in cooperation with the shaft.
The shaft is formed with two symmetrically arranged groups of V-shaped concave/convex portions (e.g. grooves), the concave/convex portions of each group being arranged at regular angular intervals on the outer circumferential surface of the shaft at an inclination angle with respect to the axial direction of the shaft so as to form two shape anisotropic portions (at which the magnetic anisotropy is determined on the basis of the shape of the shaft made of magnetostrictive material). The two coils and other two fixed resistors are connected so as to form a bridge circuit. An exciting oscillator is connected to two junction points of the bridge circuit and a differential amplifier is connected to the remaining two junction points of the bridge circuit. Therefore, when a torque is applied to the shaft, under the condition that current is passed through each of these magnetizing and detecting coils, since each inductance of the two coils changes on the basis of magnetostrictive effect at the shape anisotropic portions (e.g. grooves), it is possible to detect the magnitude of a torque applied to the shaft on the basis of change in the inductance of the detecting coil.
The above-mentioned magnetostrictive torque detecting apparatus can be mounted on an industrial robot for effecting automated grinding work, for instance. In more detail, a plurality of shape anisotropic portions are formed on the outer circumferential surface of a motor shaft provided with a grinding stone at a free end thereof, and a detection signal indicative of torque is applied to a robot controller to control the motor speed, for instance, so that the pressure of the grinding stone against a workpiece to be ground can be appropriately determined.
In the above-mentioned torque detecting apparatus, since the shaft to be measured is usually made of a relatively high magnetostrictive material, it is possible to detect magnetostrictive components of the shaft at the shape anisotropic portion at high sensitivity and in non-contact fashion. In addition, it is possible to obtain stable detection output signals from a static torque to relatively high-speed revolution torque without fluctuations in output level when the shaft is being rotated.
In the prior-art magnetostrictive torque detecting apparatus, however, since the shaft to be measured is usually made of only an Fe-Al alloy (magnetostrictive material), there exists a problem in that it is impossible to obtain stable linear torque-output characteristics over a wide torque range. That is, although excellent linear characteristics can be obtained in a low-torque range, the detection output is inevitably saturated in a high-torque range. This is because the yield point of the shaft made of magnetostrictive material is low and therefore the shaft is plastically deformed microscopically.
Further, when the shape anisotropic portions are formed by cutting, there exists a problem in that stress concentration easily occurs at the shape anisotropic portions when a high torque is applied to the shaft, thus deteriorating the linear torque-output characteristics of the detecting apparatus.
Further, when the concave/convex shape anisotropic portions are simply formed in rectangular cross section shape, there exists a problem in that cracks are easily produced at the shape anisotropic portions when the shaft is heat treated, thus reducing the productivity of the shaft.
Further, when the two symmetrically arranged V-shaped groups of concave/convex portions are formed on the same alloy layer, there exists a problem in that the symmetrical groups of concave/convex portions are subjected to the mutual inductive relationship and therefore the points at which the maximum inductance can be obtained are offset from the central positions of the two symmetrical groups of concave/convex portions, thus preventing the output drift voltage at zero-torque point due to axial displacement of the shaft relative to the coils from being minimized.